Histological Structure of Lymphoid Organs

1. Histological Structure of Lymphoid Organs DR RANIA GABR

2. Objectives • Understand the location of lymphatic organs. • Discuss the microscopic features of Lymph Node. • Discuss the microscopic features of Spleen. • Discuss the microscopic features of Thymus. • Discuss the microscopic features of Tonsils

3. Lymphoid Tissue • Lymphoid tissue is connective tissue chch by rich supply of lymphocytes. • It is found either 1- Free in regular CT 2-Surrounded by capsules, forming the “lymphoid organs” • Very little cytoplasm so stain dark blue with H&E. • Rich network of reticular fibrils produced by fibroblasts.

4. Lymphoid System Basics • Two main tissue architecture types: • Diffuse: uniform appearance • Follicular: consists of lymphoid follicles • Two types of lymphoid tissues: • Encapsulated: connective tissue capsule • spleen, thymus, lymph nodes • Unencapsulated (or partly encapsulated) • Tonsils, Peyer’s patches, lymphoid nodules in GI tract, respiratory tract, urinary & reproductive tracts

5. 2 Types of Lymphoid Organs • Central (primary) lymphoid organ: where lymphoid cells undergo maturation • T cells in thymus • B cells in bone marrow • Peripheral (secondary) lymphoid organ: where functional lymphocytes go including: 1- lymph nodes 2- spleen, 3- Peyer’s patches, 3- lymphoid nodules of GI and other tracts

7. Lymphoid Follicles • Nodules of densely packed lymphocytes located in all peripheral lymphoid tissues. Most lymphocytes are B cells. • Two distinct areas: 1- Mantle– darker stained, mainly small, resting lymphocytes 2- Germinal center – (defines “secondary” or “reactive” lymphoid follicles): lighter stained, larger, activated B cells

8. Lymph follicle: • Mantle = cap (dark) • Germinal center (light)

10. Lymph Nodes • Present throughout the body, along lymph vessels • Numerous in axilla, groin, cervical area and thoracic/abdominal mesenteries • Filter lymph before it returns to vasculature • Hilum: concave side, arteries, nerves enter; veins and efferent lymph vessels leave the organ • Afferent lymph vessels enter convex surface

12. Covered by a capsule which extends to form Trabeculae. • Divided into outer cortex and inner medulla. • OUTER CORTEX contains: Lymphatic nodules with germinal center • INNER MEDULA contains: Medullary Cords and Medullary Sinus

13. Medullary cords • Are branched, cordlike extensions of lymphoid tissue arising from the paracortex. They contain primarily B- lymphocytes and often plasma cells and macrophages. • Medullary cords are separated by dilated spaces , frequently bridged by reticular cells and fibers , called Medullary sinuses • They contain lymph , lymphocytes, macrophages, sometimes granulocytes if the lymph node is draining an infected organ

15. Lymph nodeCT --- Connective tissueC --- CortexM --- MedullaP --- ParacortexLN --- Lymph NodeT ---TrabeculaeMS --- Medullary SinusMC --- Medullary Cords